SYMPTOM:
The main blower continuously runs even when there is no call for heat and the fan switch on the thermostat is set to auto. The blower is not wired for continuous fan operation.
SOLUTION:
Normally this indicates an open circuit in the flame rollout or high limit switch circuit. Remove wires from limit and put a jumper between them. If this stops the fan replace the limit. A limit normally fails due to a blocked air filter, be sure to check it. Otherwise check all the flame rollout switches the same way. An open flame rollout could indicate a crack in the heat exchanger causing the the flames from the burner to roll out the front. Be sure to inspect for this. Always do a temperature rise check after replacing limit. Replace limit with same temperature rating as old. Temperature is written usually something like L220-30, which is 220F opening temperature and a closing temperature of 30F less or 190F.